The pressing of explosive charges by means of hydraulic presses under high pressures of up to 1000 bar and higher represents the most important process for shaping high-explosive explosive charges, besides casting. However, while plastic-bonded explosive charges produced by casting contain only at most 90 wt. % explosive, a higher percentage of explosive, equaling 95 wt. % or higher, can be reached in the case of pressed plastic-bonded explosive charges.
In plastic-bonded explosive charges, thermoplastics or curable plastics, in which the crystalline explosive particles are embedded, are used as the binder for the crystalline explosive. The charge molding is then produced from the granular explosive and plastic by pressing.
Due to the above-mentioned high percentage of explosive and the use of high explosives, such as Octogen, pressed, plastic-bonded explosive charges have a high energy content. Therefore, they are used mainly for hollow charges and similar shaped charges.
The commercially available explosive/plastic binder granular products for producing pressed charges contain especially polyurethanes as well as fluoropolymers as the plastic binder. Thus, a granular product containing a hexafluoropropylene-vinylidene fluoride polymer known under the trademark "VITON A" and another granular product with a thermoplastic polyurethane binder under the trademark "ESTANE" as the plastic binder are commercially available.
However, at high percentages of Octogen of 95 wt. % or more, the pressed explosive charges produced from these granular products are extremely sensitive and therefore do not meet the requirements imposed in terms of the safety of ammunition, e.g., against bullet impact and fire.
U.S. Pat. No. 4,050,968 discloses an explosive composition comprising an explosive such as RDX (Hexogen), HMX (Octogen), or perchlorate salts. This patent proposes an explosive content from about 50% to 95% at column 2, line 17. However, the binder content is more than 5% of the composition. In practice the binder content is much higher, namely about 18% according to the typical compositions set forth in column 5, line 63 to column 6, line 5. In this example, the castable explosive composition has an HMX-content of 80.5% or less (see table at bridging columns 5 and 6). The binder is an acrylate copolymer containing a plasticizer.
U.S. Pat. No. 4,050,968 is clearly directed to castable charges (see inter alia column 1, line 8) and such castable charges cannot have a high charge density (i.e. 97% of the theoretical maximum density). The maximum density of a cast charge, with a solids content of more than 90%, especially more than 94%, is in fact below 92%. This physical limit relates to the fact that the maximum density obtained by casting is achieved with the so-called most dense cubic or hexagonal sphere packing. In such sphere packing voids between the grains are unavoidable. This presents a physical barrier as to density. Further, for casting, the composition must freely flow. To gain a freely flowing composition, the binder must be able to form a liquid film between the grains of the explosive. Accordingly with the castable compositions as proposed by U.S. Pat. No. 4,050,968, a high binder content is necessary. U.S. Pat. No. 4,050,968 is silent about the density of the charge compared with the theoretical maximum density.
U.S. Pat. No. 4,428,786 concerns a mixture with 90 to 97% by weight of a powerful explosive compound, such as octogen, and 3 to 10% by weight of a stabilizing agent. Tetrafluoroethylene mentioned in column 1, lines 65, 66 has a high shore hardness, so that an explosive charge containing this binder will be extremely sensitive. The drop hammer test and the friction peg test stated in column 6, line 63 to column 7, line 10 are standard tests which explosives have to fulfill for obtaining transportation permission at all. That is, these are very rough tests and are not comparable with the much more sophisticated tests as the Cookoff test and the other sensitivity tests described in the report of the Naval Surface Warfare Center in Dahlgren, Va., mentioned below.
U.S. Pat. No. 4,842,659 discloses an explosive composition containing nitropropyl (NP) compounds and cellulose acetate butyrate (CAB) as binder, that is a composition which is similar to DAX-2 and PAX-2A according to table 1 of the report of the Naval Surface Warfare Center, mentioned below. As can be seen from this report, PAX-2 failed the Slow Cookoff test, because of explosion (table 5) and showed a considerably lower armor penetration at stand-offs of 2, 5 and 7 CDs as the charge of the present invention, that is PBXP-31.
U.S. Pat. No. 2,999,744 uses a polysiloxane as shown in column 3, lines 3 to 6 and 25 to 30. This is a polysiloxane with Si--O--Si bridges, that means formed by a condensation reaction. According to column 3, line 53, the polysiloxane has a Shore A hardness of 90. This polysiloxane has about the same hardness as "Viton A" used as comparison binder by the inventors (see table I below). This patent also does not disclose a pressed explosive charge, but a plastic explosive. A plastic explosive is formed by simply mixing the binder and the explosive with each other to an homogeneous mass. In contrast to that a pressed charge is formed by mixing the binder, the explosive and a solvent, and after mixing removing the solvent to form a granulate, and pressing the granulate to the charge with a high pressure of about 2000 to 3000 bars. Teachings of U.S. Pat. No. 2,999,744 relating to the inert portion (binder etc.) may have little or no value in the pressed charge field.
U.S. Pat. No. 4,088,518 discloses a thermosetting cross-linked silicone binder (column 2, lines 28, 29). This cross-linking is attained by a condensation reaction, that is by forming H.sub.2 O-molecules, so that 2 silicone atoms are bridged by an oxygen atom in the silicone. U.S. Pat. No. 4,088,518 (Kehren) uses a similar polysiloxane as U.S. Pat. No. 2,999,744 (Eckles). That is, according to column 3, line 34 to column 4, line 60 of Kehren the polysiloxanes are formed by a condensation reaction as follows: ##STR1## Consequently, Kehren also obtains charges having a high mechanical strength, as a high modulus of elasticity and a high compressive strength. This is expressively stated in column 2, line 8 ("exceptional mechanical strength"). Furthermore, according to the examples the crushing strength of Kehren's explosive is in the range of about 60 to 350 bars, that is 3 to 35 N/mm.sup.2, so that Kehren's charge has about the same strength as the comparison charges according to Table II below.
The prior art explosive charges which meet high safety standards tend to have high binder content and lower explosive content with lower performance. Those explosive compositions with high explosive content do not meet the high safety standards which are desired.